1. Field of the Invention
The present invention relates generally to an information recording/reproducing method, and more particularly to a method of recording a digital signal on a recording medium and reproducing the digital signal therefrom.
2. Description of the Related Art
A holographic memory system, which is one of information recording/reproduction systems mentioned above, records a digital signal (hereinafter called "data") on a holographic memory medium (a photo-refractive crystal material such as LiNbO.sub.3 or the like) and reproducing the data therefrom. The system is capable of recording and reproducing data in a form of two-dimensional plane page, and moreover capable of recording and reproducing over a large number of pages. An exemplary configuration of the system is illustrated in FIG. 1.
Referring to FIG. 1, an encoder 11 converts time series recording data sequences to be recorded into a "page" in a holographic memory medium 1. In other words, the encoder 11 rearranges the time series recording data into a data matrix corresponding to a two-dimensional unitary plane page as a predetermined unitary recording region, for example, in a matrix of vertically 480 bits and horizontally 640 bits (480.times.640) to produce unitary page data sequence. The unitary page data sequence is sent to a spatial light modulator (SLM) 12.
The SLM 12 optically modulates an irradiated signal beam in accordance with the unitary page data sequence-from the encoder 11 in modulation processing units of vertically 480 pixels.times.horizontally 640 pixels corresponding to the unitary page, and leads a modulated beam resulting therefrom to a lens 13. More specifically, the SLM 12 passes a signal beam therethrough in response to a logical value "1" in the unitary page data sequence, which is an electrical signal, and blocks the signal beam in response to a logical value "0,". to achieve electrical-optical transducing in accordance with the: respective bit contents in the unitary page data to produce a modulated signal beam as an optical signal representing the unitary page sequence.
Such a modulated signal beam is input to the holographic memory medium 1 through the lens 13. The holographic memory medium 1 is also irradiated with a reference beam having an angle from a predetermined reference line (hereinafter, called the "incident angle .beta.") orthogonal to the optical axis of the beam carrying the optical signal, other than the modulated signal beam.
When the modulated signal beam and the reference beam are simultaneously incident on the holographic memory medium 1, both beams interfere with each other within the holographic memory medium 1 to produce an interference pattern which is recorded in the holographic memory medium 1, thereby recording the data in the holographic memory medium 1. Also, by inputting the reference beam with a different incident angle .beta., data can be recorded in the holographic memory medium 1 in units of predetermined three-dimensional recording regions including a plurality of pages of two-dimensional data.
For reproducing recorded data from the holographic memory medium 1, unlike recording, no signal beam is input to the holographic memory medium 1, but the reference beam only is input at the same incident angle .beta. as that used during the recording. In this way, diffracted light from an interference pattern recorded in the holographic memory medium 1 is led to a lens 21.
The diffracted light reaching the lens 21 passes through the lens 21, and impinges on a CCD (Charge-Coupled Device) 22 having a light receiving area of vertically 480 pixels.times.horizontally 640 pixels. Each of the pixels in the light receiving area of the CCD 22 corresponds to each pixel on a recording plane in the holographic: memory medium 1, so that the CCD 22 transduces the brightness of the, incident light in each pixel area into the magnitude of an electrical signal level, i.e., generates an analog electrical signal indicative of a level corresponding to the luminance of incident light, and supplies the analog electrical signal to a decoder 23 as a read signal.
The decoder 23 has a function of digitizing the read signal or performing binary determination, and recognizes a logical value "1" when the read signal has a level higher than a slice level serving as a threshold value, and a logical value "0" when lower than the level to produce a digital signal which carries the values thus recognized. In addition, the decoder 23 performs a reverse conversion of conversion performed in the encoder 11 to the digital signal to produce time series reproduced signal.
The holographic memory system thus configured is capable of recording and reproducing three-dimensional data including a temporal element to and from the holographic memory medium 1 by changing an incident angle .beta. of a reference beam at arbitrary time-intervals, as well as recording and reproducing planar two-dimensional data in vertical and horizontal dimensions.
However, a variety of factors such as dust and stain on each optical elements, crosstalk, interference fringes and so on may cause the light intensity to spatially and temporally fluctuate, resulting in fluctuations due to noise in amplitude occurring in the output of the CCD, i.e., a read signal, other than a change in amplitude due to data itself. When the output of the CCD 22 is converted to data of "1" or "0" based on a fixed slice level in the decoder 23, data, read errors may occur owing to amplitude fluctuations.
By way of example, assume that an image carrying a two-dimensional data matrix as illustrated in FIG. 2 is recorded in the holographic memory medium 1.
In FIG. 2, a white portion represents data "1" while a black portion represents data "0". If an irregular luminance illustrated In FIG. 3 is superimposed on the pattern image carrying such data, a read signal is produced on the basis of a pattern image as illustrated in FIG. 4. In this case, an amplitude value of a read signal in a data "1" portion is affected by "dark" or lower luminance irregularity to become smaller.
In FIG. 4, when a portion free from a "dark" region due to the irregular luminance indicated by an arrow A is sliced in the horizontal direction, and is represented as a change in the output level of light received by the CCD 22 corresponding to the sliced portion, i.e., a change in the level of the read signal, a waveform as shown in FIG. 5 is derived.
In FIG. 5, by determining whether the read signal is "1" or "0" based on a slice level defined by a median value between a maximum value and a minimum value of the read signal, recorded data "1, 0, 1, 0, 1, 0, 1. 0, 1, 0, 1, 0, 1, 0, 1" can be correctly reproduced as reproduced data "1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1."
However, if a portion including a "dark" region due to irregular luminance indicated by an arrow B is sliced in the horizontal direction, and a read signal from that portion is determined whether it is "1" or "0" with the same slice level as FIG. 5, recorded data is not correctly reproduced because a level corresponding to the "dark" region is lower than the slice level as shown in FIG. 6, so that the reproduced data represents "1, 0, 1, 0. 1, 0, 0, 0, 0, 0. 0. 0, 1, 0, 1, " thus resulting in a read error.
While FIG. 6 shows an example where the level of a read signal corresponding to data "1" becomes lower due to a "dark" region of this irregular luminance to result in a read error, the level of a read signal corresponding to data "0" may become higher due to a "light" region of irregular luminance to cause a read error, as shown in FIG. 7.
Also, as shown in FIGS. 8 and 9, data cannot be correctly reproduced if the entire level of a read signal fluctuates with an offset due to the irregular luminance to cause a read error.
In the case shown in FIGS. 8, 9, while a large offset may result in the level of a read signal exceeding an upper limit of data "1" or falling below a lower limit of data "0" the level is limited by the respective upper limit or the lower limit.